The present disclosure generally relates to energy coupling in optical systems, and more particularly, to optical systems operating in near-field configurations.
Radiation energy can be coupled between two objects at least in part by evanescent waves when the objects are in a near-field configuration, i.e., they are separated from each other by a distance approximately equal to or less than one wavelength of the radiation. One unique feature of near-field coupling is that a beam can be focused to a spot that has a size smaller than the minimum size set by the diffraction of the radiation. An optical system based on the near-field coupling can produce an optical numerical aperture greater than unity.
The near-field coupling may be used in various applications. For example, the spatial resolution of an imaging system can be increased beyond the diffraction limit, such as in a near-field microscope. An optical storage system may also achieve a high areal storage density by implementing a specially-constructed optical head to operate in a near-field configuration with respect to the storage media.
A radiation signal that has at least a portion evanescently coupled from one surface to another is sensitive to the variation of the spacing between the surfaces. Hence, it may be desirable to maintain the spacing at a desired constant within a predetermined tolerance range, or to filter or modify the received signal to minimize or remove any effect caused by the spacing variation. Therefore, a mechanism of measuring the spacing variation between the two surfaces in the near-field configuration is desirable, particularly when such measurements need to be done in real time during the operation of a near-field optical system.
One embodiment of such a near-field optical system has an optical head, an optical signal-selecting device, a radiation detector, and a processing circuit. The optical head has an optically transparent interfacing surface that couples radiation energy at least in part by evanescent fields to and from a reflective surface that is spaced from the interfacing surface by less than one wavelength. The optical signal-selecting device is in an optical path of received radiation energy from the reflective surface by the optical head to select a radiation signal from the received radiation energy. This selected radiation signal includes an evanescent-coupled component and varies with a spacing between the interfacing surface and the reflective surface.
The radiation detector is disposed to receive and convert the radiation signal into an electrical signal radiative of the spacing between the interfacing surface and the reflective surface. The processing circuit is coupled to process this electrical signal to determine the spacing.
The above system may be configured as an inspection system to determine the degree of the flatness of optical disks, magnetic disks, or other flat surfaces. It may also be incorporated in a near-field optical disk drive to monitor the spacing between the optical head and the disk during operation.